The present invention relates to a technique that can be effectively applied to the correction of D.C. offsets and the adjustment of gains in a variable gain amplifier circuit provided in the reception circuitry of a radio communication system and to a high gain amplifier circuit which successively amplifies reception signals with a plurality of amplifier circuits in multiple stage connection, for instance a technique that can be effectively utilized in a wireless local area network (LAN) system and high frequency semiconductor integrated circuits (ICs) and baseband large scale integrated circuits (LSIs) used therein.
A wireless LAN system or a cellular phone is usually provided with a high gain amplifier circuit which comprises a plurality of low-pass filters and variable gain amplifiers alternately connected in a multiple stage configuration and amplifies I signals and Q signals to respectively predetermined amplitude levels while eliminating unnecessary waves in order to amplify down-converted and demodulated reception signals (I signals which are components in phase with a fundamental wave and Q signals which are components orthogonal thereto) to predetermined levels and delivering them to a baseband circuit.
In a reception circuit wherein mixers for down-converting and demodulating reception signals, a high gain amplifier circuit at a stage subsequent thereto and variable gain amplifiers of the individual stages constituting the high gain amplifier circuit are D.C. coupled to each other, any D.C. offset occurring in the output of a mixer would be amplified by the variable gain amplifiers constituting the high gain amplifier circuit, and at the same time any D.C. offset in a variable gain amplifier at a prior stage would be significantly amplified by another variable gain amplifier at a subsequent stage, resulting in variations in the D.C. voltages of the outputs of the amplifiers.
Known formulas proposed for correcting D.C. offsets in variable gain amplifiers constituting a high gain amplifier circuit for amplifying reception signals in a radio communication system include, for instance, one by which output signals are subjected to A/D conversion by an A/D converter in a state in which the differential input terminals of variable gain amplifiers are short-circuited to each other, a value to make those outputs “0” is generated and subjected to D/A conversion by a D/A converter, and the converted value is added to input signals to the variable gain amplifiers (see Patent Reference 1), and another by which D.C. offsets are detected from carrier leaks arising from re-modulation of demodulated I and Q signals (see Patent Reference 2).
[Patent Reference 1]
Japanese Unexamined Patent Publication No. 2002-217762
[Patent Reference 2]
Japanese Unexamined Patent Publication No. 2001-160835